fish are usually easier to keep, but there are a few exceptions.
in a 100 gallon and for your first tank you'll want either a FO (fish only) or FOWLR (fish only with live rock).
the only thing else you NEED is a hydrometer, and salt. but at that size i strongly reccomend getting a protein skimmer.
you'd you be more intersted in getting a but load of smaller fish or a few larger fish?,
that sort affects what else you need.